Flow visualization on SFR wire-wrapped 19 pin bundle and comparison with CFD analysis

The wire-wrapped pin bundle is the most commonly adopted form of the fuel assembly in SFRs. It is challenging to measure the local flow velocity field experimentally because of its narrow channel and complex internal shape. In this study, the refractive indices of the fluid and the test-section are matched and the flow field in the SFR wire-wrapped 19-pin bundle is visualized using optical measurement techniques such as PIV/LDV and compared with the RANS-based computational fluid dynamic (CFD) analysis. According to the turbulence intensity and the pressure drop measurements, it is observed that the flow regime changes from the transition regime to the fully turbulent regime when the Re number is more than 13,000. In addition, the pressure drop measurement results are compared with the CFD analysis for various turbulence models, and it is found that the BSL-RSM model predicts the experimental results the most accurately. The velocity distribution obtained in the edge sub-channel using PIV is also compared with the CFD results. As a result, the flow and vortex shapes are very similar to each other qualitatively. However, some discrepancies are observed quantitatively in the region where the channel thickness is narrow. The main reason is considered to be attributed to the pin location error, the refraction of light and the velocity averaging due to the thickness of the laser sheet incident on the thin channel during the experiment. Further investigation is on-going for analysis of flow in interior subchannel.